22 SEPTEMBER 2017 • VOL 357 ISSUE 6357 1219 SCIENCE sciencemag.org
The spacecraft is gone. The mission is not. Last week, after 13 years of explora- tion, NASA’s Cassinispacecraftplunged into the upper reaches of Saturn’s at- mosphere at 123,000 kilometers per
hour and melted away. Beaming back its last
measurements to mission control at the Jet
Propulsion Laboratory (JPL) in Pasadena, California, the orbiter stayed
alive for 30 seconds longer than expected during its fiery dive. Finally, at
4:55 a.m. local time, the radio signals
stopped: Cassini’s aluminum and
polymer skeleton had vaporized.
The spacecraft’s demise, necessitated by dwindling fuel and a need to protect
two of Saturn’s 62 moons from potential microbial contamination from Earth, brought
forth a global outpouring of sentiment. In
Pasadena, in a sleepy predawn haze, the mission’s extended web of researchers watched
the signal fade out. Many have known each
other for decades, their families growing
up together. “My scientific life is tied to this
spacecraft, to this mission,” says Luciano Iess,
a planetary scientist at the Sapienza University of Rome who has led Cassini’s radio experiment since 1990.
Although pathos ruled for a day, Cassini’s
scientists are eager to get back to work. The
spacecraft has already revolutionized understanding of gas giants and, with its discoveries of hydrogen-rich water plumes on
Enceladus and methane lakes on Titan, the
potential for life to exist beyond the classic “habitable zone” (see sidebar, p. 1220).
But its final 22 orbits, which NASA dubbed
the Grand Finale, made it clear that there
is much more to learn about Saturn’s atmosphere and interior. “What I can tell you is
many of our models are too simple or just
out and out wrong,” says Linda Spilker,
Cassini’s project scientist at JPL.
Starting in April, the spacecraft began
threading the gap between Saturn and its
rings once a week, an approach thought to
be too dangerous earlier in the mission. At
first, Cassini wielded its large radio antenna
as a shield to protect itself from stray ring
particles. The precaution was unnecessary;
the gap was almost empty. “We’re all still
struggling to understand that,” says Matt
Hedman, a planetary scientist at the Uni-
versity of Idaho in Moscow.
Cassini captured just enough ring particles to directly measure them for the
first time and reveal the trace chemicals
in them beyond water ice, says Nicolas
Altobelli, Cassini’s project scientist at the
European Space Agency in Madrid. As the
doomed probe dipped its toes into Saturn’s
atmosphere during its final orbits,
the ring grains disappeared, surprising his team. Altobelli isn’t
sure whether an unknown atmospheric dynamic had cleared them
away—or whether the instrument
didn’t work properly outside the
vacuum it was designed for.
Six of the final orbits were tailored for
Cassini’s radio science experiment. A radio telescope on Earth listened for tiny
Doppler shifts in the craft’s signal as the
spacecraft sped up and slowed down,
tugged by the planet and its rings. Previously, Cassini only orbited outside of the
rings, so scientists couldn’t distinguish between the masses of the planet and rings.
But once it began threading the gap, scientists could untangle the two measures,
which could help resolve a debate about
the age of the rings and inform models of
By Paul Voosen
A fiery finish to Cassini’s long run at Saturn
Spacecraft’s last orbits could improve understanding of the planet’s rings and interior
In September, Cassini
made a last portrait of Saturn
before its fatal plunge.
“Saturn is not a small version of Jupiter.
The planets are distinct and unique.”
Jonathan Fortney, University of California, Santa Cruz